1. Field of the Invention
The present invention relates to a semiconductor device such as a nitride semiconductor laser device, and to a method for the fabrication thereof.
2. Description of Related Art
As compared with AlGaInAs- and AlGaInP-based semiconductors, nitride semiconductors such as GaN, AlGaN, GaInN, AlGaInN, and mixed crystals thereof are characterized by their larger band gaps Eg and their being direct-transition semiconductor materials. These properties make nitride semiconductors attractive materials for semiconductor light-emitting devices such as semiconductor lasers that emit light in a short-wavelength region from ultraviolet to green and light-emitting diodes that cover a wide light emission wavelength range from ultraviolet to red. Thus, nitride semiconductors are believed to find wide application in high-density optical disks and full-color displays to environmental and medical fields.
Moreover, nitride semiconductors offer higher thermal conductivity than GaAs-based and other semiconductors, and are thus expected to find application in high-temperature, high-output devices. Furthermore, nitride semiconductors do not require the use of such materials as correspond to arsenic (As) in AlGaAs-based semiconductors or cadmium in ZnCdSSe-based semiconductors or source materials therefor such as arsine (AsH3), and are thus expected as environment-friendly compound semiconductor materials.
One conventional problem with nitride semiconductors is that, in the fabrication of nitride semiconductor devices as exemplified by nitride semiconductor laser devices, the yield, i.e., the ratio of the number of properly working nitride semiconductor laser devices to the total number of those produced on a single wafer, is extremely low.
The reasons are as follows. To separate the individual nitride semiconductor laser devices produced on a wafer from one another, first, the wafer is cleaved in the direction perpendicular to the direction of the resonators of the nitride semiconductor laser devices so that the wafer is split into bars having resonator end faces formed at the cleavage surfaces. Next, to further separate the individual nitride semiconductor laser devices now lying together on separate pieces of a nitride semiconductor substrate in the form of bars cleaved from one another, the thus produced bars need to be further split in the direction parallel to the direction of the resonators. Here, when the wafer is split into bars, if the nitride semiconductor substrate is of a nitride semiconductor such as n-type GaN, both the nitride semiconductor substrate and the nitride semiconductor growth layer laid on top thereof have cleavage surfaces in the direction perpendicular to the direction of the resonators, and thus they can be cleaved easily.
However, since a nitride semiconductor substrate of a nitride semiconductor such as n-type GaN has a hexagonal crystal structure and thus does not have cleavage surfaces in the direction parallel to the direction of the resonators, it is difficult to split the bar further into discrete nitride semiconductor laser devices. Thus, the splitting here causes chipping and cracking, and in addition cleavage in unintended directions, leading to a low yield.
According to one conventionally proposed solution to this problem, after a nitride semiconductor growth layer is laid on top of a substrate, grooves are formed, with a dicing machine, from the surface of the nitride semiconductor growth layer halfway into the thickness of the substrate, then the substrate is polished to become thinner, then scribe lines are drawn on the surface of the grooves formed with the dicing machine, and then a load is applied to the substrate. This helps separate individual nitride semiconductor laser devices from one another at a good yield (see Japanese Patent Application Laid-Open No. H5-315646).
Another cause for a low yield is development of cracks. Such cracks may originate from the nitride semiconductor growth layer laid on top of the substrate. Specifically, when a nitride semiconductor laser device is produced, a nitride semiconductor growth layer is laid on top of a substrate, and this nitride semiconductor growth layer is composed of different types of film, such as films of GaN, AlGaN, and InGaN. Here, the different films forming the nitride semiconductor growth layer have different lattice constants, and thus cause lattice mismatch, resulting in development of cracks. To avoid this, according to one conventionally proposed method, a processed substrate is used, and, after a nitride semiconductor growth layer is formed on top thereof, depressions are formed on the surface of the nitride semiconductor growth layer instead of the surface being flattened. This helps reduce cracks (see Japanese Patent Application Laid-Open No. 2002-246698). By the use of this method, it is possible to reduce, for example, cracks resulting from mismatching among the lattice constants of the individual films that constitute the nitride semiconductor growth layer formed on top of the substrate.
When a nitride semiconductor laser device is produced by the use of the technique disclosed in Japanese Patent Application Laid-Open No. 2002-246698 mentioned above, the nitride semiconductor growth layer is structured, for example, as shown in FIG. 19.
Specifically, on the surface of a processed substrate 10 (see FIGS. 18a and 18b) formed of an etched n-type GaN substrate or the like, a nitride semiconductor growth layer 11 is formed that is composed of, for example, the following layers laid on top of one another in the order named: a 1.0 μm thick n-type GaN layer 100; a 1.5 μm thick n-type Al0.062Ga0.938N first clad layer 101; a 0.2 μm thick n-type Al0.1Ga0.9N second clad layer 102; a 0.1 μm thick n-type Al0.062Ga0.938N third clad layer 103; a 0.1 μm thick n-type GaN guide layer 104; a multiple quantum well active layer 105 consisting of three 4 nm thick InGaN well layers and four 8 nm thick GaN barrier layers; a 20 nm thick p-type Al0.3Ga0.7N evaporation prevention layer 106; a 0.05 μm thick p-type GaN guide layer 107; a 0.5 μm thick p-type Al0.062Ga0.938N clad layer 108; and a 0.1 μm thick p-type GaN contact layer 109. The multiple quantum well active layer 105 has the layers thereof laid in the following order: a barrier layer, a well layer, a barrier layer, a well layer, a barrier layer, a well layer, and a barrier layer.
In crystallography, it is common practice, when an index indicating a plane or orientation of a crystal is negative, to express the index with an overscore placed above the absolute value thereof. In the following descriptions, since such notation is impossible, a negative index is expressed by a minus sign “−” followed by the absolute value thereof.
In the present specification, a “dissimilar substrate” denotes a substrate other than one formed of a nitride semiconductor. Examples of dissimilar substrates include sapphire substrates, SiC substrates, and GaAs substrates.
A “processed substrate” denotes a substrate having engraved regions and ridges formed on the surface of a nitride semiconductor substrate or on the surface of a nitride semiconductor thin film laid on the surface of a nitride semiconductor substrate or dissimilar substrate. In the following descriptions, the layer formed by laying on top of one another the layers doped with Mg, namely the p-type Al0.3Ga0.7N evaporation prevention layer 106, the p-type GaN guide layer 107, the p-type Al0.062Ga0.938N clad layer 108, and the p-type GaN contact layer 109, will be referred to as the “p-layer”.
On the processed surface of the processed substrate 10, the nitride semiconductor growth layer 11 is laid by MOCVD (metal organic chemical vapor deposition) to form a nitride semiconductor wafer having depressions on the surface of the nitride semiconductor growth layer 11 as shown in FIGS. 18a and 18b. In FIGS. 18a and 18b, plane orientations are indicated together.
In FIG. 18b, an n-type GaN substrate is used as the substrate 10, and there are formed engraved regions 16 and ridges 19 in the shape of stripes in the [1-100] direction by dry etching such as RIE (reactive ion etching). The engraved regions are 5 μm wide and 3 μm deep, and the interval between two adjacent engraved regions is 15 μm. On top of the thus etched substrate 10, the nitride semiconductor growth layer 11 structured as shown in FIG. 19 is produced by a growth method such as MOCVD.
Disappointingly, however, when nitride semiconductor laser devices were produced by the use of the technique disclosed in Japanese Patent Application Laid-Open No. 2002-246698 mentioned above, with an n-type GaN substrate used as a substrate 10 and with a nitride semiconductor growth layer 11 epitaxially grown on top of this n-type GaN substrate by MOCVD or the like, it was indeed possible to reduce cracks, but it was not possible to significantly increase the yield. Specifically, by the use of the technique disclosed in Japanese Patent Application Laid-Open No. 2002-246698, a plurality of nitride semiconductor laser devices were produced, of which 100 were randomly extracted and subjected to the measurement of the FWHMs (full widths at half maximum) of their FFPs (far field patterns) in the horizontal and vertical directions. Here, those nitride semiconductor laser devices which exhibited FFPs of which the FWHMs were within ±1° of the design value thereof were evaluated as acceptable. The result was that the number of nitride semiconductor laser devices that exhibited FFPs of which the FWHMs fulfilled the requirement was 30, a very low yield.
This is because leaving depressions on the nitride semiconductor growth layer 11 degrades the flatness of the film. Degraded flatness causes variations in the thicknesses of the individual layers within the nitride semiconductor growth layer 11, and thus causes the characteristics of the nitride semiconductor laser devices to vary from one individual to another, reducing the number of devices of which the characteristics fall within the required ranges. Thus, to increase the yield, it is necessary not only to reduce cracks but also to enhance the flatness of the film.
Also measured was the surface flatness within the surface of the nitride semiconductor wafer formed as shown in FIGS. 18a, 18b, and 19. The result of the measurement obtained by measuring the surface flatness in the [1-100] direction is shown in FIG. 20. The measurement was conducted under the following conditions: measurement length: 600 μm; measurement time: 3s; probe pressure: 30 mg; and horizontal resolution: 1 μm/sample. From the graph of FIG. 20, the level difference between the highest and lowest parts of the surface within the 600 μm wide region measured was found to be 200 nm.
This difference in flatness results from, as shown in FIG. 18b, the thicknesses of the individual layers of the nitride semiconductor growth layer 11 laid on top of the substrate 10 varying from one position on the wafer to another. Consequently, the characteristics of nitride semiconductor laser devices vary greatly depending on where within the wafer surface they are produced, and the thickness of the Mg-doped p-layer (corresponding to the sum of the p-type layers laid as the p-type Al0.3Ga0.7N evaporation prevention layer 106 to the p-type GaN contact layer 109 shown in FIG. 19), which thickness greatly affects the characteristics of nitride semiconductor laser devices, varies greatly from one position to another within the substrate surface.
When a ridge structure as a current constriction structure is formed, the ridges are left in the shape of 2 μm wide stripes, and the rest is etched away by a dry etching technique using an ICP (inductively coupled plasma) machine or the like. Thus, if the p-layer thickness before etching varies from one position to another within the wafer surface, the remaining p-layer thickness after etching, which most greatly affects the characteristics of nitride semiconductor laser devices, varies from one position to another within the wafer surface. As a result, not only does the layer thickness vary from one nitride semiconductor laser device to another, even within one nitride semiconductor laser device, the remaining p-layer thickness may be almost zero in some parts and quite large in other parts. These variations in the remaining p-layer thickness affect the lasing lives of nitride semiconductor laser devices and, as described above, the characteristics thereof such as the FWHMs of FFPs.
This large distribution of the layer thickness within the wafer surface is considered to result from the fact that the growth rate of the film that is epitaxially grown at the ridges on the processed substrate including a nitride semiconductor substrate varies under the influence of the engraved regions, resulting in uneven growth rates within the wafer surface.
Specifically, when epitaxial growth is started on a substrate 10 having engraved regions 16 formed thereon as shown in FIG. 21a, at an initial stage of growth, the parts of the nitride semiconductor thin film that grow on floor portions 124 and side portions 126 of the engraved regions 16, called the engraved region growth portions 122, only partially fill the engraved regions 16. At this stage, the parts of the nitride semiconductor thin film that grow on the surface of top portions 123 of the ridges 19, called the top growth portions 121, grows with the surface of the nitride semiconductor thin film kept flat.
As the epitaxial growth of the nitride semiconductor thin film proceeds from the above-described stage shown in FIG. 21a to a stage shown in FIG. 21b, where the engraved region growth portions 122, i.e., the parts of the nitride semiconductor thin film that grow on the floor portions 124 and side portions 126 of the engraved regions 16, almost completely fills the engraved regions 16, these parts become coupled, via growth portions 125, to the top growth portions 121, i.e., the parts of the nitride semiconductor thin film that grow on the surface of the top portions 123 of the ridges 19. At this stage, the source material atoms and molecules (for example, Ga atoms) that have attached to the surface of the nitride semiconductor thin film that has grown on the top portions 123 of the ridges 19 is made to migrate or otherwise move, by heat energy, into the growth portions 125 and engraved region growth portions 122. This migrating movement of atoms and molecules occurs extremely unevenly within the wafer surface, and the movement distance varies within the wafer surface. As a result, as shown in FIG. 21b, the flatness of the surface of the top growth portions 121 is degraded.
This flatness of the nitride semiconductor thin film is degraded also in the [1-100] direction under the influence of the unevenness of the nitride semiconductor substrate itself, such as the distribution of the off-angle within the wafer surface and the distribution of the substrate curvature within the wafer surface; the unevenness of the epitaxial growth rate within the substrate surface; the unevenness of the engraving process within the substrate surface; and other factors. Specifically, the time required for the engraved regions 16 to be filled varies in the [1-100] direction, and thus, where they are filled early, atoms and molecules of the source materials from which the nitride semiconductor thin film is formed migrate or otherwise move from the top growth portions 121 of the ridges 19 into the growth portions 125 or engraved region growth portions 122. Thus, where those atoms and molecules migrate to, it takes more time to form the nitride semiconductor thin film, with the result that the nitride semiconductor thin film formed in the engraved regions 16 becomes thicker. On the other hand, where the engraved regions 16 are incompletely filled, no atoms or molecules of the source materials from which the nitride semiconductor thin film is formed move from the top growth portions 121 of the ridges 19 into the engraved regions 16; even if they do, it takes less time to form the nitride semiconductor thin film. Thus, the nitride semiconductor thin film formed in the engraved regions 16 is thinner than where the engraved regions 16 are filled earlier.
In a so-called supply-governed state, i.e., a state in which the growth rate is governed by the flux or the like of the atoms and molecules supplied to the wafer surface, if atoms and molecules of the source materials from which the nitride semiconductor thin film is formed migrate or otherwise flow into the engraved regions 16, since the flux of the source material atoms and molecules supplied to the entire wafer surface is fixed, the top growth portions 121, where the nitride semiconductor thin film grows on the top portions 123 of the ridges 19, become thinner. Otherwise, i.e., if no atoms or molecules of the source materials from which the nitride semiconductor thin film is formed migrate or otherwise flow into the engraved regions 16, the top growth portions 121, where the nitride semiconductor thin film grows on the top portions 123 of the ridges 19, become thicker.
As a result, the thickness of the top growth portions 121 on the top portions 123 of the ridges 19 varies within the wafer surface, degrading the flatness of the surface of the nitride semiconductor thin film. Thus, to enhance the flatness, it is necessary to suppress the formation of the nitride semiconductor thin film as a result of atoms and molecules of the source materials from which the nitride semiconductor thin film is formed migrating or otherwise moving from the top growth portions 121 of the ridges 19 into the growth portions 125 or engraved region growth portions 122.
Moreover, it has been found that, when nitride semiconductor laser devices are produced by the technique disclosed in Japanese Patent Application Laid-Open No. 2002-246698 mentioned above, if electrodes are formed in depressions on the surface of the nitride semiconductor growth layer 11, current leak paths develop in the depressions, making it impossible to obtain a normal I-V characteristic. Usually, an insulating film such as SiO2 is formed above depressions, and electrodes are formed further on top. The presence of the depressions here, however, causes the insulating film to be formed unevenly on the surface, leaving a large number of small cracks, very thin regions, small holes (pits), and the like. Thus, through unevenly formed parts of the insulating film, current leaks.
On the other hand, it has also been found that, when individual nitride semiconductor laser devices produced on a nitride semiconductor substrate by the use of the technique disclosed in Japanese Patent Application Laid-Open No. H5-315646 mentioned above are separated from one another, since it is after a nitride semiconductor growth layer is laid on top of the nitride semiconductor substrate that grooves are formed by the use of a dicing machine, the nitride semiconductor growth layer may be internally damaged, degrading the characteristics of the nitride semiconductor laser devices.